Does a copy of a spell count as being cast for the purpose of prowess triggers?

Prowess triggers whenever you cast a noncreature spell (CR 702.107a). The word "cast" is the critical term here. When a spell is copied — for example, by a card like Twincast or Reverberate — the copy is placed directly onto the stack without being cast. CR 706.10 explicitly states that a copy of a spell is not cast.

Because prowess specifically checks for the casting event (the moment a spell is cast and placed on the stack via the casting process described in CR 601.2), a copy that simply appears on the stack without going through that process does not satisfy prowess's trigger condition.

It is worth noting that the original spell that was copied was cast, so it would have already triggered prowess when it was cast. Only the copy itself fails to trigger prowess.

Concrete example: You control a creature with prowess and cast Lightning Bolt (triggering prowess once). Your opponent uses Reverberate to copy your Lightning Bolt. The copy of Lightning Bolt is placed on the stack, but since it was not cast, it does not trigger prowess again.

Relevant rules: CR 702.107a defines prowess and its trigger condition; CR 601.2 defines what it means to cast a spell; CR 706.10 clarifies that copies are not cast.